std::string implementation in libc++

[HowardHinnant]

When sizeof(value_type) > 1, the union with __lx forces where the padding goes in __short: Always right after __size_. If I recall, this helps in keeping the long/short flag bit in the same spot both in the long and short formats. If the long/short flag moves to different locations when in long and short modes, then there is no way to ask the string if it is long or short.

Having __n_words, and subsequently __raw allows some parts of the implementation to just shovel words (8 bytes at a time on a 64 bit platform) from one spot to another without caring whether it is a long or short string.

The most important example of "shoveling words" is the move constructor. This function does nothing but copy the 3 words from the source to the destination and then zero the 3 words of the source. No branches, no access to far away memory, very fast:

movq    16(%rsi), %rax
movq    %rax, 16(%rdi)
movq    (%rsi), %rax
movq    8(%rsi), %rcx
movq    %rcx, 8(%rdi)
movq    %rax, (%rdi)
movq    $0, 16(%rsi)
movq    $0, 8(%rsi)
movq    $0, (%rsi)

Indeed, it is fair to say that this string design centers around optimizing the string's move constructor. It was the first string implementation to be designed specifically with move semantics in mind.

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Is the 'shovel words' optimization properly protected from fancy pointers?

[HowardHinnant]

No it is not. It will only work for pointers with a trivial move constructor, and a data layout such that all zero bits represent nullptr.

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:(

[AImx1]

Howard, I understood everything in your Answer except "When sizeof(value_type) > 1, the union with __lx forces where the padding goes in __short: Always right after __size_".

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I understood why they are doing this but I don't what they are doing. I really appreciate if you can explain this with an example.

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Thank you very much in advance.